This invention relates to the beneficiation of mineral values by froth flotation and more particularly to the beneficiation of coal by froth flotation.
Many valuable minerals, such as coal, are beneficiated by froth flotation. Coal is an extremely valuable natural resource in the United States because of its relatively abundant supply. Energy shortages together with the availability of abundant coal reserves has created a new interest in the use of coal as an alternate or primary energy source. As a result, great efforts are being taken to make coal and related solid carbonaceous materials equivalent or better sources of energy than petroleum or natural gas. In this context, numerous techniques have been and are being explored to make coal cleaner burning, more suitable for burning and more readily transportable.
Coal must be cleaned because it contains substantial amounts of sulfur, nitrogen compounds and mineral matter, including significant quantities of metal impurities. During combustion these materials enter the environment as sulfur dioxides, nitrogen oxides and compounds of metal impurities. If coal is to be accepted as a primary or alternate energy source, it must be cleaned to prevent pollution of the environment.
Accordingly, physical, as well as chemical, coal cleaning (beneficiation) processes have been extensively explored. In general, physical coal cleaning processes involve grinding the coal to release the impurities, wherein the fineness of the coal generally governs the degree to which the impurities are released. However, because the costs of preparing the coal rise exponentially with the amount of fines, there is an economic optimum in size reduction. Moreover, grinding coal even to the finest sizes is not effective in removing all impurities.
Based on the physical properties that effect the separation of the coal from the impurities, physical coal cleaning methods are divided into four general categories: gravity, flotation, magnetic and electrical methods.
Chemical coal cleaning techniques are in a very early stage of development. Known chemical coal cleaning techniques include, for example, oxidative desulfurization of coal (sulfur is converted to a water-soluble form by air oxidation), ferric salt leaching (oxidation of pyritic sulfur with ferric sulfate), and hydrogen peroxide-sulfuric acid leaching.
A recent promising development in the art of chemical coal beneficiation is disclosed in U.S. Pat. No. 4,304,573 incorporated herein by reference. In summary, according to this chemical coal beneficiation froth flotation process, coal is first cleaned of rock and the like and pulverized to a fine size. The pulverized coal, now in the form of a water slurry, is then contacted with a mixture comprising a polymerizable monomer, polymerization catalyst and fuel oil. The resultant surface treated coal is highly hydrophobic and oleophilic and is thus readily separated from unwanted ash and sulfur using oil and water flotation separation techniques.
In this flotation process and in others which employ anionic collectors, the presence of large amounts of cations, however, in the process water is deleterious to overall efficiency of the process. Even in flotation processes, such as coal flotation processes, which do not utilize these collectors, the presence of these cations increases the ash content and lowers the fusion temperature of the ash which results in undesirable increased slag and caking when the coal is burned.
It is desirable therefore to provide a flotation process which avoids these disadvantages and furthermore results in cleaner and enhanced mineral, e.g. coal, recoveries.